Two main categories of process have been proposed for the manufacture of such fuel gases as described below with reference to coal as the starting material.
The first category of process consists of gasification of coal with steam and substantially pure oxygen to provide a raw gas having as its main constituents hydrogen, carbon oxides and methane. The raw gas is subjected to processing steps such as carbon monoxide shift, methanation, removal of sulphur compounds, removal of carbon dioxide and/or physical separation methods to produce a fuel gas of the desired calorific value and combustion characteristics. Examples of proposed processes for the production of SNG from coal include the following:
(a) a gasifier containing slowly descending bed of coal either with slagging of the ash (British Gas process) or without slagging of the ash (Lurgi process), cooling of the product and washing to remove tar and aromatic components, removal of sulphur compounds and some carbon dioxide by liquid washes, and methanation to produce pipeline quality SNG. PA0 (b) an entrained-flow gasifier fed with a coal-water slurry (Texaco process), cooling of the tar-free gas, partial carbon monoxide shift, removal of sulphur compounds and some carbon dioxide and methanation to produce pipeline quality SNG. PA0 (c) a fluid bed gasifier (IGT's U-Gas process) followed by a process sequence as in (b) above. PA0 (i) a hydrogen stream which is used to hydrogenate the solid carbonaceous material, PA0 (ii) a methane containing fuel gas stream, and PA0 (iii) a waste nitrogen stream which may be heated and expanded through a turbine to recover useful energy.
The second category of process depends on the direct hydrogenation of coal to produce methane. Hydrogenation is in principle a thermally more efficient method of producing methane than the above described processes comprising oxygen/steam gasification followed by methanation. This is because the hydrogenation step is less exothermic than methanation and the oxygen/steam gasifier needed to make the necessary hydrogen has only to gasify a part of the coal supply. This type of process has been developed mainly by British Gas and by the Institute of Gas Technology (Hygas Process).
Most hydrogenation-based SNG plants designed to date have contained at least two separate process streams:
a coal hydrogenation stream producing SNG from hydrogen and coal and leaving as a by-product a residual carbonaceous material known as "char"; and
a hydrogen production stream producing hydrogen by oxygen/steam gasification of residual char from the hydrogenator, carbon oxide shift and acid gas removal,
an optional third steam reforming stream to make additional hydrogen from aromatics produced in the hydrogenator may also be included.
The two types of process described above both utilize substantially pure oxygen as a reactant with carbonaceous material. Oxygen is generally obtained from air by a separation technique e.g. cryogenic separation which is a source of considerable expense in the manufacturing process. Furthermore, strict precautions must be taken with the oxygen-containing stream and oxidation reaction to control the explosive potential of the gases.
It is an object of the present invention to provide a process for the production of fuel gas from carbonaceous material which does not require the use of substantially pure oxygen.